This invention relates to a battery charging circuit, and more particularly to such a circuit where two transformers are used, one being a main transformer for supplying main charging current, and the other being a boost transformer for supplying trickle charge current. The advantages of the invention are both economy and efficiency, allowing the use in an efficient manner of standard transformers that are mass produced.
Battery charging circuits including multiple transformers, or transformers having multiple secondary windings, are well known.
Cronvall U.S. Pat. No. 2,431,312 discloses a charging circuit using choke coils to control the charging current produced by the transformer 2. In Cronvall there does not appear to be a separate transformer supplying trickle charge to the battery.
Jensen U.S. Pat. No. 3,090,904 discloses a battery charging circuit where the coils 31 and 32 supply the charge current. After a certain time has elapsed, a timer closes a switch to place capacitance in the resonant circuit to increase the charge voltage near the end of the charge cycle to provide "gassing" of the battery.
Chait U.S. Pat. No. 3,193,750 discloses the use of two transformers. The transformer 52 establishes a control voltage such that when the battery voltage exceeds the control voltage, the transformer 12 supplies a trickle charge through a variable resistor. Chait does not use a separate boost transformer for supplying the trickle charge.
Conforti U.S. Pat. No. 4,460,863 provides a trickle charge through an LED and resistor, but not from a separate transformer.
Rhyne U.S. Pat. No. 3,843,918, and Jardine U.S. Pat. No. 3,217,228 are further examples of battery chargers using a single transformer with multiple secondary windings and resistance in the secondary to limit the charging current. Other examples of known battery charger circuits are disclosed in U.S. Pat. Nos. 2,875,396, 3,278,823, 3,659,181, 4,233,553, and 4,313,078.
The major advantages of the present invention over the prior art are economy and efficiency. Generally, the present invention utilizes two transformers. One is the main transformer supplying power to produce the main charging current for charging the battery. The second is a boost transformer for supplying the trickle charge. Preferably, the main transformer is of a center tap, ferro-resonant type, and the boost transformer is of high impedance. Initially, as the battery is being charged, the main transformer supplies nearly all of the power for charging the battery, the boost transformer supplying only a very small percentage of the power, for example, about 3%. As the battery is charged to its rated voltage, less power is supplied from the main transformer, so that an increasingly higher percentage of power is supplied from the boost transformer. Once the battery reaches rated voltage, the battery continues to be trickle charged with greater power from the boost transformer and with very little power supplied from the main transformer.
Hence, the battery charger circuit of this invention allows efficient use of standard transformers that are mass produced. The main and boost transformers are of standard type, so it is not necessary to manufacture a special transformer for a particular charging requirement. For example, such special transformers might require specified numbers of windings or taps in the secondary that are nonstandard. Such special purpose transformers are expensive to manufacture. The battery charger circuit of the present invention is also efficient in the use of a high impedance boost transformer to supply the trickle charge without requiring resistance as the primary means for limiting the current as with some prior art battery charging circuits. Such resistance dissipates power and provides low efficiency. By using a boost transformer with high reactive impedance, the amount of energy lost by use of limiting resistors is much reduced. This is true even where some resistance is used in the secondary, because the current in the secondary is small due to the high transformer reactive impedance.
The boost transformer and its associated circuitry can also be used as a retrofit kit for existing chargers for supplying trickle charge. The booster kit itself has a low VA rating, and is therefore relatively inexpensive for the purpose of supplying trickle charge.
These and other advantages of the invention are apparent from the drawing and detailed description to follow.